Apparatus and method for generating electric energy in an electric power transmission system

ABSTRACT

Apparatus and method for generating electric energy in an electric power transmission system includes an AC cable including a core, wherein a portion of the core is partially surrounded by an apparatus including a ferromagnetic body and an electrically conducting winding. The ferromagnetic body extends along a longitudinal axis and has, in a cross section taken along said longitudinal axis, a shape defined by an arc. The electrically conducting winding is wound around the ferromagnetic body to form turns in planes substantially perpendicular to the arc.

The present invention relates to electric energy generation in anelectric power transmission system.

Electric energy generation in an electric power transmission system canbe useful for supplying ancillary electric devices.

For example, ancillary electric devices can be part of a monitoringsystem for surveying parameters of the electric power transmissionsystem, which typically comprises electric conductors, junctions andterminations.

For example, WO 99/58992 discloses a power cable monitoring systemcomprising one or more transducers distributed along a sea cable. Powerfor the transducers and the transducer signal converters is suppliedthrough electric conductors positioned in the data cable or throughbatteries.

The applicant observes that the use of batteries for feeding amonitoring system is expensive and may need maintenance. Indeed,batteries may need to be substituted, for example because exhausted, andthis entails additional cost. In some instance, for example when thecable and the associated monitoring system is positioned in a remote orenvironmentally challenging location, the maintenance operation impliesexpensive means (e.g. for submarine cables) and/or the interruption ofthe power transmission (e.g. for high voltage cables).

At http://www.saprem.com/08_(—)3_(—)2_luminosaconductor.pdf, the companySAPREM S.A. de Preformados Metálicos discloses a warning light beaconfor signalling the presence of cables suspended in high and mediumtension overhead lines, which uses the field created by the conductor asa power supply. The warning light beacon has a transformer divided intwo parts to make easier the installation on the conductor.

The Applicant faced the technical problem of providing a cable systemwith an apparatus for generating electric energy useful, for example,for supplying ancillary devices of the cable system, said apparatusbeing able to exploit a local energy source.

In particular, the Applicant faced the technical problem of generatingelectric energy in an electric power transmission system comprising aninsulated conductor, individually sheathed, and optionally provided witha metallic screen.

The problem is particularly felt in the presence of at least twoindividually sheathed and insulated conductors, laid down adjacent to orin contact with each other.

The Applicant found that this problem can be solved by collecting energyfrom the magnetic field produced by the alternating current (AC) flowingalong an electric conductor and transforming the collected energy intoelectric energy by means of a ferromagnetic body and an electricallyconducting winding wound around it. The Applicant surprisingly foundthat a ferromagnetic body having a cross section defined by an arc,which surrounds the electric conductor for only part of its angularextension, enables power values of practical utility to be obtained.Moreover, the Applicant surprisingly found that such an arc shapedferromagnetic body enables power values of practical utility to beobtained even when applied on an insulated sheathed conductor, outsidewhich—due to the insulation layer, to the protective layer(s) and to theoptional screening layer(s)—the magnetic field produced by thealternating current (AC) flowing along the electric conductor is muchweaker than outside an aerial bare conductor.

Accordingly, in a first aspect the present invention relates to a cablesystem comprising an AC cable comprising a core and an apparatus forgenerating electric energy, the apparatus comprising

-   -   an arc shaped ferromagnetic body extending along a longitudinal        axis of the AC cable, and    -   at least one electrically conducting winding wound around the        ferromagnetic body to form turns in planes substantially        perpendicular to the arc;        wherein the ferromagnetic body is operatively associated with        the AC cable to surround a portion of said core.

In a second aspect the present invention relates to an apparatus forgenerating electric energy in an electric power transmission system, theapparatus comprising:

-   -   an arc shaped ferromagnetic body extending along a longitudinal        axis; and    -   at least one electrically conducting winding wound around the        ferromagnetic body to form turns in planes substantially        perpendicular to the arc.

For the purpose of the present description and of the appended claims,except where otherwise indicated, all numbers expressing amounts,quantities, percentages, and so forth, are to be understood as beingmodified in all instances by the term “about”. Also, all ranges includeany combination of the maximum and minimum points disclosed and includeany intermediate ranges therein, which may or may not be specificallyenumerated herein.

The expression “in planes substantially perpendicular to the arc” canindicate that the turns formed by the winding lie in planes that mightdeviate from planes perpendicular to the arc of ±5°, preferably, ±1°.Preferably, the planes can deviate of ±0.35°. The more perpendicular arethe planes of the turns to the arc the higher is the coupling efficiencyof the magnetic field into the winding.

In the present description and claims, the term “core” is used toindicate an electric conductor surrounded by at least one insulatinglayer and at least one protective sheath. Optionally, said core furthercomprises at least one semiconductive layer. Optionally, said corefurther comprises a metal screen.

Preferably, the ferromagnetic body is fixed upon a portion of anexternal surface of an outmost layer, typically a sheath, of the core.

In a preferred embodiment of the invention the cable comprises at leasttwo cores individually insulated, individually sheathed and, optionally,individually screened.

The core(s) can be single phase core(s).

Advantageously, the arc shape is such as to surround the core by leavinga gap not higher than 10 mm.

In the case of a core having an external diameter ranging from 4 to 20cm, the internal radius of the arc shaped ferromagnetic body can be offrom 2 to 10 cm.

Advantageously, the cable system comprises at least one furtherapparatus for generating electric energy, according to the second aspectof the invention, wherein the ferromagnetic body of the at least onefurther electric apparatus is operatively associated with the AC cableto surround a further portion of said core.

Advantageously, the AC cable comprises at least one further core. Inthis case, the ferromagnetic bodies of the apparatuses for generatingelectric energy can be operatively associated on only one of the coresof the cable. Alternatively, part of the ferromagnetic bodies can beoperatively associated with one of the cores and another part of theferromagnetic bodies can be operatively associated with the othercore(s). In an embodiment, the core and the at least one further corecomprise each an insulated, individually sheathed, electric conductor.

In an embodiment, the core and the at least one further core are laiddown with at least part of their outer surface adjacent to or in contactwith each other.

The cable can comprise more than two cores. In AC systems, the cableadvantageously is a three-phase cable. The three-phase cableadvantageously comprises at least three insulated single phase cores.

The three insulated cores can be protected together within a singlesheath or they can be individually protected within three separatesheaths.

The three insulated cores may be on a planar configuration or in atrefoil configuration. In the planar configuration the three insulatedcores have the longitudinal axes thereof lying substantially in a sameplane. In the trefoil configuration, the three insulated cores arereciprocally positioned in such a way that, in a cross section takenalong their longitudinal axes, they have, as a whole, a trefoil shape.

The invention is particularly advantageous in the case of at least twoinsulated cores, individually sheathed (and, optionally, individuallyscreened), that are laid down with at least part of their outer surfaceadjacent to or in contact with each other. For example, the invention isparticularly advantageous in the case of three insulated conductors,individually sheathed (and, optionally, individually screened),positioned in a trefoil configuration.

Indeed, the ferromagnetic body with a cross section defined by an arcenables the apparatus of the invention to be fixed upon a free portionof the outer surface of one of the at least two cores (that is, on aportion that is not adjacent to or in contact with the outer surface ofthe other core(s)).

The AC cable can be a low, medium or high voltage cable.

The term low voltage is used to indicate voltages lower than 1 kV.

The term medium voltage is used to indicate voltages of from 1 to 35 kV.

The term high voltage is used to indicate voltages higher than 35 kV.

The AC cable may be terrestrial, submarine or of the windmill type.

The terrestrial cable can be at least in part buried or positioned intunnels.

Terrestrial, submarine and windmill cables advantageously comprise atleast one core comprising an electric conductor surrounded by aninsulating layer and at least one protective sheath.

The present invention can be applied also to cores wherein the conductoris bare, such core configuration being typically used in aerial cables.As the aerial cables are used in overhead plants, the main insulatingelement of the bare conductor is formed by the surrounding air.

Aerial cables can comprise an aluminium-steel electric conductor.

When the cable is aerial, the ferromagnetic body can be fixed on aportion of an external surface of the bare electric conductor.

For arc shaped ferromagnetic body extending along a longitudinal axis itis intended a body that, in a cross section taken along saidlongitudinal axis, has a shape defined by an arc that can extend for anangle lower than 360°.

Said arc advantageously extends for an angle lower than 300°. Said arcadvantageously extends for an angle at least equal to 45°.

In an embodiment, the ferromagnetic body has a substantiallysemi-circular cross section. That is, the ferromagnetic body issubstantially semi-cylindrical.

Preferably, said arc extends for an angle at least equal to 180°. Forexample, in a preferred embodiment, said arc extends for an angle ofabout 270°.

The ferromagnetic body can be made of a monolithic metal or of a metalin form of a plurality of lamellae.

Preferably, for a body of 10 cm in length by 1 cm in thickness, thewinding comprises a number of turns of from 400 to 800.

The winding preferably has a diameter in the range of 0.2 mm to 3 mm,more preferably, between 0.4 mm to 1.5 mm.

The ferromagnetic body preferably has a length of from 6 cm to 40 cm.This range can enable to obtain a good compromise between the needs ofhaving a compact apparatus and the need of generating useful powerlevels.

The above mentioned ranges of number of turns, winding diameter, andferromagnetic body length are exemplary ranges that allow power levelsof practical utility to be obtained from the apparatus of the invention.

Advantageously, the winding is made of an insulated metallic conductor,as a copper wire, preferably with enamelled insulation.

The cable system typically further comprises cable junctions andterminations.

The cable system can comprise a plurality of (that is, more than one) ACcables.

Advantageously the cable system further comprises at least one electricdevice operatively associated with the apparatus for generating electricenergy so as to be electrically supplied by it.

The at least one electric device can be operatively associated with thecore of the AC cable.

The at least one electric device can be associated with a junctionand/or with a termination.

The at least one electric device can be any ancillary electric device.For example, it can be a monitoring node for monitoring at least oneparameter of the cable system. For example, the monitoring device cancomprise a partial discharge monitoring unit adapted to detect possiblepartial discharges occurring in the cable system.

It is noted that the apparatus of the invention generates electricenergy by exploiting a local source (i.e., the magnetic field generatedby the alternating current (AC) flowing along an electric conductor ofthe cable system) that is not constant and continuous with time. Indeed,the intensity of the induced magnetic field depends from the intensityof the current flowing along the electric conductor, which may bedifferent between day and night, between various seasons of the year,between working days and non-working days, and similar.

Accordingly, the apparatus is advantageously associated with a batteryunit for storing the electric energy generated by the apparatus itself,for example when it exceeds the energy necessary to supply the at leastone electric ancillary device.

Moreover, the at least one electric device advantageously is a low powerconsumption electric device.

Preferably, the at least one electric device is adapted to alternativelyoperate in a sleeping mode and in an active mode so as to reduce powerconsumption.

In an embodiment of the invention, the cable system comprises an ACcable comprising a core, a plurality of apparatuses according to thesecond aspect of the invention and a monitoring system for monitoringparameters of the cable system. The monitoring system comprises acentral unit and a plurality of monitoring nodes adapted to be placed atdifferent monitoring points of the cable system. The ferromagnetic bodyof each of the plurality of apparatuses is operatively associated withthe AC cable, so as to partially surround a corresponding portion ofsaid core. The plurality of monitoring nodes is connected to theplurality of apparatuses so as to be electrically supplied by them.Advantageously, the monitoring nodes are connected in cascade. Moreover,each monitoring node is advantageously adapted to alternatively operatein a sleeping mode and in an active mode, wherein, during each activemode, each monitoring node is adapted to:

-   -   acquire a value of at least one of said parameters and to        process the acquired value so as to generate corresponding        output data;    -   to receive output data from an upward node of the cascade, if        any; and

to send to a downward node, if any, the output data received from saidupward node and the output data generated by the monitoring node itself,a last monitoring node of the cascade being adapted to send said outputdata to the central unit, the central unit being adapted to collect theoutput data coming from the monitoring nodes.

In the present description and claims the expression:

-   -   “upward monitoring node” with respect to a given monitoring node        is used to indicate a node that precedes said given monitoring        node with respect to a direction of propagation of data towards        a central unit. The expression “upward monitoring node” can be        used to indicate a node that, with respect to said given        monitoring node, is farther from the central unit;    -   “downward monitoring node” with respect to a given monitoring        node is used to indicate a node that follows said given        monitoring node with respect to a direction of propagation of        data towards a central unit. The expression “downward monitoring        node” can be used to indicate a node that, with respect to said        given monitoring node, is closer to the central unit;    -   “last monitoring node”, with respect to a cascade of monitoring        nodes, is used to indicate the last monitoring node of the        cascade with respect to a direction of propagation of data        towards a central unit. The expression “last monitoring node”        can indicate the node closest to the central unit;    -   “first monitoring node”, with respect to a cascade of monitoring        nodes, is used to indicate the first monitoring node of the        cascade with respect to a direction of propagation of data        towards a central unit. The expression “first monitoring node”        can indicate the node farthest to the central unit;    -   “sleeping mode” is used to indicate an idle mode of a node        wherein the node does not perform any operation of data receipt,        data transmission and data acquisition;    -   “active mode” is used to indicate an operating mode of a node        wherein the node performs operations of data receipt, data        transmission and data acquisition;    -   “cascade” is used to indicate a plurality of monitoring nodes        connected in series so that the output data of one are        transmitted to the next, with respect to a direction of        propagation of data towards a central unit;    -   “data link” is used to indicate a path through which at least        two devices (e.g., nodes, central unit . . . ) can transmit data        to each other.

Advantageously, the monitoring nodes are adapted to operatealternatively in the sleeping mode and in the active mode according tosynchronized time frames.

Advantageously, the time frames are synchronized in such a way that themonitoring nodes pass from a sleeping mode to an active mode insequence, one after the other.

Preferably, the time frames are synchronized in such a way that eachmonitoring node starts to operate in an active mode before (preferablyright before) the upward monitoring node starts sending to it the outputdata.

Preferably, the time frames are synchronized so as to minimize thewaiting time for receiving output data from an upward monitoring node.

Advantageously, the monitoring nodes are connected to each other incascade through a plurality of data links.

The data links can be wired or wireless, the latter being preferred.

In case of wired link, the data link can be an optical fiber link(comprising at least one optical fiber) or an electrical link(comprising at least one electrical wire, preferably at least twoelectrical wires).

In case of optical fiber link, each monitoring node advantageouslycomprises electro-optical converters.

In case of wireless link, the data link can be a radio frequency (RF)link.

In case of wireless link, each node advantageously comprises at leastone antenna and at least one RF transceiver.

The data links each can have a length of from 1 m to 1600 m, ifwireless, of from 1 m to 40 km, if wired with optical fiber, or of from1 m to 1 km, if wired with electrical wire.

Preferably, the data links have each a length of from 20 m to 200 m, ifwireless or wired with electrical wire, or of from 1 m to 1 km, if wiredwith optical fiber.

In view of reducing power consumption, these ranges of data link lengthsare given in order to enable low power data transmissions betweenmonitoring nodes (as, for example, RF wireless data transmissions withirradiated power levels equal to or lower than 100 mW, preferably equalto or lower than 1 mW).

Moreover, the above mentioned ranges of data link lengths areadvantageously selected so as to obtain a good compromise between costand reliability.

Indeed, shorter data link lengths could imply a larger number of nodesand, thus, higher costs. On the other hand, they can imply higherreliability because they can enable to collect more information and, incase of failure, to reduce the risk of missing important informationabout a point of the cable system (the information may be obtained by anearby node).

The central unit itself can be adapted to operate alternatively in asleeping mode and in an active mode, with a time frame synchronized withthe time frame of the last monitoring node of the cascade. In this case,the central unit is adapted to receive the output data from the lastmonitoring node of the cascade only when operating in an active mode.

Typically, the monitoring nodes comprise each at least one sensor. Thesensor may be adapted to detect at least one cable parameter, forexample cable temperature, ambient temperature, ambient humidity, waterflooding, cable current, screen current, cable voltage, fire, gas,aperture of access doors, cable strain, cable displacement, vibrations,and similar.

Advantageously, the monitoring system comprises a processing stationadapted to process the output data coming from the monitoring nodes.This allows to provide an operator with useful information indicative ofthe operating conditions of the cable system.

Advantageously, the processing station is a remote station.

The central unit is advantageously adapted to act as interface betweenthe last monitoring node of the cascade and the processing station andto send output data received by the last node of the cascade to theprocessing station. The central unit can be connected to a modem or arouter for communicating with the remote station.

In a third aspect the present invention relates to a method ofgenerating electric energy in an electric power transmission systemcomprising an AC cable comprising a core, the method comprising thesteps of:

-   -   operatively associating an apparatus with the core, said        apparatus comprising an arc shaped ferromagnetic body extending        along a longitudinal axis of the AC cable and at least one        electrically conducting winding, the at least one electrically        conducting winding having two electric terminations and being        wound around the ferromagnetic body to form turns in planes        substantially perpendicular to the arc; and    -   making an alternating electric current flowing along the core        thereby generating a magnetic field around the core that        generates a voltage difference at the two electric terminations        of the at least one electrically conducting winding.

The features and advantages of the present invention will be madeapparent by the following detailed description of some exemplaryembodiments thereof, provided merely by way of non-limiting examples,description that will be conducted by making reference to the attacheddrawings, wherein:

FIG. 1 schematically shows an embodiment of an apparatus according tothe invention;

FIG. 2 schematically shows, in cross section, an exemplarily cablesystem according to an embodiment of the invention;

FIG. 3 schematically shows, in cross section, an exemplarily cablesystem according to another embodiment of the invention;

FIG. 4 schematically shows, in a perspective view, the cable system ofFIG. 3;

FIG. 5 schematically shows, in cross section, an example of an AC cablefor a cable system according to the invention;

FIG. 6 schematically shows a cable system according to anotherembodiment of the invention;

FIGS. 7 a and 7 b schematically respectively show a perspective view anda cross sectional view of a closed ring ferromagnetic body folded up onitself, which has been tested by the Applicant;

FIG. 8 schematically shows an embodiment of a monitoring node of thecable system of FIG. 6;

FIG. 9 schematically shows an example of an auto-synchronization processperformed by a monitoring node of the cable system of FIG. 6;

FIGS. 10 a and 10 b schematically show two exemplarily flowchartsoutlining the main actions carried out by the monitoring nodes of thecable system of FIG. 6 in order to maintain synchronization betweennodes while alternatively operating in a sleeping mode and in an activemode;

FIG. 11 schematically shows an example of a data frame that can be usedto transmit data between the monitoring nodes of the cable system ofFIG. 6.

FIG. 1 shows an embodiment of an apparatus 200 according to theinvention comprising an arc shaped ferromagnetic body 210 extendingalong a longitudinal axis L and an electric winding 220, which is woundaround the body 210 to form turns in radial planes, substantiallyperpendicular to the arc.

Even if in FIG. 1 only one electric winding 220 is shown, the apparatusmay also comprise more than one winding suitably connected in series orin parallel, depending on the needs.

Moreover, even if in the embodiment shown in FIG. 1 the electric winding220 is wound around the ferromagnetic body in a single layer of turns,the electric winding 220 can be wound around the ferromagnetic body soas to form more than one layer of turns, one above the other.

FIG. 2 schematically shows, in cross section, a cable system comprisingan AC cable comprising a single core 14 and the apparatus 200, whereinthe apparatus 200 is fixed upon the single core 14, so as to surround alongitudinal portion of an external surface of the core 14 for part ofthe angular extension of the core about its longitudinal axis. As shownin FIG. 2, the apparatus 200 is fixed upon the core 14 so that itslongitudinal axis L is substantially coincident with the longitudinalaxis of the core 14.

The ferromagnetic body 210 advantageously has a cross section of shapeand size such as to fit the profile of the external surface of the core14. Gaps of some mms between the internal surface of the apparatus 200and the external surface of the core 14 are tolerated.

The apparatus 200 can be fixed upon the core 14 by a suitable binder, asuitable strap or, when it surrounds the core 14 for an angularextension higher than 180°, by elastic clamping.

The core 14 advantageously comprises an individually insulated andsheathed electric conductor.

FIG. 5 shows, in cross section, an exemplarily high voltage cablecomprising a core 14 with an individually insulated and sheathedelectric conductor.

In the example, the core 14 comprises a central metal conductor 105; abinder 110 made of a semi-conductive tape; a conductor screen 115 madeof a semi-conductive polymer; an insulation layer 120 made, for example,of cross-linked polyethylene (XLPE); an insulation screen 125, also madeof a semi-conductive polymer; semi-conductive water barriers 130 and 140made for example of a semi-conductive hygroscopic tape; a screen 145made of a metal in form of, e.g., tapes and/or wires; a sheath 150 ofhigh-density polyethylene (HDPE); and a protective coating 155,typically semi-conductive.

The apparatus 200 can be fixed upon the core 14, to partially surround aportion of the protective coating 155.

In the embodiment shown in FIGS. 2-4, the ferromagnetic body 210 has asemi-cylindrical shape.

As shown in FIGS. 3 and 4, the arc shaped ferromagnetic body 210 can beadvantageous when the apparatus 200 is fixed on a core 14, individuallysheathed, which is part of a trefoil cable configuration.

A trefoil cable configuration is typically used for terrestrial,submarine and windmill high voltage cables.

In the embodiment shown in FIGS. 3 and 4, the trefoil cableconfiguration comprises three cores 14 comprising each an individuallyinsulated and sheathed electric conductor. Moreover, the apparatus 200is fixed on only one of the three cores 14. However, two or threeapparatuses 200 can be fixed on two or all three cores 14.

The Applicant found that for certain uses, as that disclosed below withreference to FIG. 6, it is sufficient that the apparatus 200 is fixed ononly one of the three cores 14.

In the apparatus 200, electric energy is generated by collecting(through the ferromagnetic body 210) the magnetic field generated by analternating current that flows along the core 14 and transforming it(through the winding 220) into a voltage difference at two electricterminations 230 of the winding 220.

The Applicant made experiments and numerical simulations in order totest electric energy generation in such an apparatus.

The Applicant started with a closed ring ferromagnetic body in aconfiguration folded up on itself as shown in FIGS. 7 a and 7 b. Indeedthis configuration is suitable to be fixed upon one of at least twoinsulated and individually sheathed electric conductors, laid down withat least part of their outer surface adjacent to or in contact with eachother.

The closed ring ferromagnetic body was 20 mm in length (along thelongitudinal axis L′) by 3 mm in thickness (t), had an internal radius(R_(int)) of 47.5 mm and a height (h) of 69.5 mm.

The ferromagnetic body was made of Magnifer®50 by the companyThyssenKrupp VDM GmbH, which is a nickel-iron alloy with a quantity ofnickel of about 48%.

Moreover, the experiments have been carried out by fixing the apparatusupon an insulated sheathed single core cable by Prysmian Cables ySistemas S.L. of the type 1*1200 Al+H 141 Cu 76/138 kV ENDESA KNE-001.

The external part of the ferromagnetic body (that one which was notdirectly in contact with the external surface of the insulated sheathedsingle core cable) was wound by a winding made of enamelled copper wireof 0.62 mm in external diameter.

The apparatus wrapped up the cable for at least 50% of its perimeter.

The tables below show the results experimentally obtained by varying thevalues of the current flowing along the cable (I_(cable), first column)and the number of winding turns. The voltage, current and powermeasurements (second, third and fourth column) have been made at the twoelectric terminations of the winding without and with a 270 Ohmsresistor connected thereto.

TABLE 1 (number of turns = 3000, open circuit without resistor)I_(cable) [A] V[V] 104 4.3 209 8.4 299 11.6 404 15.29 498 18.2 601 20.78

TABLE 2 (number of turns = 1000, open circuit without resistor)I_(cable) [A] V[V] 102 1.43 211 2.82 299 3.87 405 5.15 505 6.18 606 7.01710 7.74

TABLE 3 (number of turns = 1000, with a 270 Ohms resistor) I_(cable)Power [A] V[V] I[mA] [mW] 100 0.34 1.3 0.4 210 0.976 3.6 3.5 297 2.288.,4 19.3 405 4.55 16.9 76.7 507 6.33 23.4 148.4 608 7.56 28.0 211.7 7098.51 31.5 268.2

TABLE 4 (number of turns = 2000, with a 270 Ohms resistor) I_(cable)Power [A] V[V] I[mA] [mW] 108 0.208 0.8 0.2 195 0.517 1.9 1.0 297 1.726.4 11.0 406 5 18.5 92.6 510 9 33.3 300.0 596 12.02 44.5 535.1 697 14.0352.0 729.0

TABLE 5 (number of turns = 3000, with a 270 Ohms resistor) I_(cable)Power [A] V[V] I[mA] [mW] 100 0.13 0.5 0.1 203 0.39 1.4 0.6 304 1.13 4.24.7 402 3.75 13.9 52.1 501 7.31 27.1 197.9 603 11.08 41.0 454.7 71215.05 55.7 838.9

It is noted that the intensity of the current flowing along a cable ofan electric power transmission system may vary from low to high values(e.g., from 0-150 A to 1000-2000 A) between day and night, betweenvarious seasons of the year, between working days and non-working days,and similar. It is thus desirable that—depending on the uses—enoughenergy generation is guaranteed also at reasonably low currents (e.g.,100-150 A). In the above tests, at reasonably low cable currents (e.g.,100-150 A), power levels lower than 1 mW were obtained.

The Applicant then tested an arc shaped ferromagnetic body, inparticular a semi-cylindrical ferromagnetic body, having higher lengthand thickness values than the closed ring body previously tested.

In particular, the experiments were carried out with an apparatus havinga semi-cylindrical ferromagnetic body and a winding of enamelled copperwire of 0.62 mm in external diameter. The ferromagnetic body was 100 mmin length, 10 mm in thickness and has an internal bending radius of51.01 mm. The ferromagnetic body was made of M400-50 soft iron.

Moreover, the experiments have been carried out by fixing the apparatusupon an insulated sheathed single core cable by Prysmian Cables ySistemas S.L. of the type 1*1200 Al+H 141 Cu 76/138 kV ENDESA KNE-001(the same of the previous tests).

The following tables show the power levels (second column)experimentally obtained by varying the values of the current flowingalong the cable (I_(cable), first column), for different ferromagneticbody lengths and winding turns. The power measurements have been madewith a 47 Ohms resistor connected to the two electric terminations ofthe winding.

TABLE 6 (ferromagnetic body length of 10 cm, number of turns = 600)I_(cable) [A] Power [mW] 90 38.777 100 45.977 150 102.045 200 191.489250 291.277 300 393.404 350 553.404 400 765.957

TABLE 7 (ferromagnetic body length of 12 cm, number of turns = 550)I_(cable) [A] Power [mW] 90 17.619 100 26.215 150 53.789 200 96.530 250146.051 300 216.513 350 285.013 400 375.319

The Applicant also made experiments with a cable and an apparatus havingan arc shaped ferromagnetic body and a winding having the samecharacteristics as disclosed above with reference to table 6, with theonly difference that the ferromagnetic body had an arc shaped crosssection extending for an angle of about 270°.

The following table shows the power levels experimentally obtained byvarying the values of the current flowing along the cable (I_(cable),first column), for the semi-cylindrical ferromagnetic body of table 6,with arc shaped cross section extending for 180°, (second column) andfor the ferromagnetic body with arc shaped cross section extending for270° (third column). The measurements have been made putting the twoelectric terminations of the winding in short circuit. In order tocalculate the power generated by the apparatus, the internal resistanceof the winding was taken in account. In this case, for a 600 turnswinding, an internal resistance of 14 Ohms was taken in account.

TABLE 8 I_(cable) [A] Power [mW] - 180° Power [mW] - 270° 95.2 26.523123.704 299.2 410.027 1251.614 584 1583.720 4840.416 1002 4785.14915084.216

The experimental results of the above tables 6-8 surprisingly showedthat an arc shaped ferromagnetic body enables power levels higher than26 mW to be obtained for reasonably low cable currents (e.g., about100-150 A). In particular, power levels of from about 100 to 1250 mWwere obtained for cable currents of about 95-300 A with theferromagnetic body having an arc shaped cross section extending for270°.

Power levels of 100-200 mW can be, for example, of practical utility forsupplying a monitoring node as that disclosed below with reference toFIG. 6.

In view of what stated above (that the intensity of the current flowingalong a cable of an electric power transmission system may vary from lowto high values between day and night, between various seasons of theyear, between working days and non-working days, and similar) the factthat the arc shaped ferromagnetic body allows achieving power levels ofpractical utility for reasonably low cable currents (e.g., 100-150 A) isan important result.

The above tables 6 and 7 further show that better power levels wereobtained for a ferromagnetic body length of 10 cm and for a number ofturns of 600.

Moreover, table 8 shows that the power levels obtained with theferromagnetic body having arc shaped cross section extending for 270°are 3-5 times larger than the power levels obtained with thesemi-cylindrical ferromagnetic body (with arc shaped cross sectionextending for 180°).

The Applicant made further experiments and numerical simulations thatshowed that an arc shaped ferromagnetic body can be made of anyferromagnetic material without significantly affecting the efficiency ofthe apparatus.

This is advantageous because it allow using low cost ferromagneticmaterials.

Moreover, the use of an arc shaped ferromagnetic body allows reducingthe quantity of material used to make the apparatus and, thus,production cost with respect to a closed ring body.

In general, the experiments and numerical simulations carried out by theApplicant surprisingly showed that even though:

-   -   for an arc shaped ferromagnetic body, which surrounds the core        of an AC cable for only part of its angular extension about its        longitudinal axis, the magnetic field collection efficiency is        lower than for a closed ring ferromagnetic body, which surrounds        the core for the whole of its angular extension; and even though    -   outside an insulated sheathed conductor—due to the insulation        layer(s) and to the protective layer(s)—the magnetic field        produced by the alternating current (AC) flowing along the        electric conductor is much weaker than outside an aerial bare        conductor,        power values of practical utility (e.g., higher than 100-200 mW)        can still be obtained by the apparatus of the invention, when        operatively associated with an insulated sheathed conductor.

The apparatus 200 of the invention can be used to supply energy to anexternal electric device 100.

As shown in FIG. 1, the winding 220 has two electric terminations 230that can be connected to the external electric device 100.

The external device 100 can be, for example, a monitoring node of amonitoring system.

FIG. 6 shows a preferred embodiment of the invention comprising a cablesystem with an AC cable comprising a core 14, a plurality of apparatuses200 according to the invention, a corresponding plurality of monitoringnodes 100, a central unit 12 and a remote processing station 10,connected through a network 1.

The monitoring nodes 100 are placed in cascade at different distancesfrom the central unit 12.

The apparatuses 200 are positioned along the cable, with theirferromagnetic bodies (not shown in FIG. 6) fixed upon the core 14 topartially surround a corresponding portion of its external surface.

Each apparatus 200 is electrically connected to a corresponding one ofthe monitoring nodes 100 in order to supply it with electric energy.

Even if not shown, the cable system of FIG. 6 may also comprise two ormore cores (e.g., three cores in a trefoil configuration). In this case,the plurality of apparatuses 200 and the corresponding plurality ofmonitoring nodes 100 can be fixed on different monitoring points of onlyone of the plurality of cores, with the sensors of the monitoring nodes100 suitably placed on different monitoring points of all the cores.

Even if in FIG. 6 there are exemplarily shown five monitoring nodes 100,it will be clear that the cable system can comprise more or less thanfive monitoring nodes, depending on the needs and the length to becovered by the cable system.

An example of cable system comprises up to 256 monitoring nodes, at adistance of 50 m from each other, so as to cover a length of 12.8 km.

The monitoring nodes 100 are placed in cascade at different distancesfrom the central unit 12.

Preferably, the monitoring nodes 100 are equidistant. Moreover, thedistance between the last node (node 5) and the central unit 12 ispreferably the same as the distance between two nodes. This allows thedesign of the monitoring nodes, as far as concern transmission/receptionparameters, to be the same.

In the above-mentioned example, the distance between two nodes andbetween the last node and the central unit 12 is of 50 m.

The central unit 12 is preferably positioned at the end of the cascadeof monitoring nodes 100. The central unit may be positioned at a manhole(for example underground) or at a shunting substation (which can beunderground or above the ground, for example in a building), whereinconnection to a main power source and/or the remote processing station10 is typically easier.

The central unit 12 can be connected to a modem or to a router (notshown), for example through a wired connection.

The central unit 12 acts as interface between the last node (e.g., node5) and the remote processing station 10.

The central unit, especially when connected to a main power source, canbe operated always in active mode.

In the embodiment of FIG. 8, the monitoring node 100 comprises anelectronic board 160 and a plurality of sensors 169.

The electronic board 160 comprises a programmable low powermicroprocessor 162, a backup battery 164, a plurality of connectors 166for the sensors 169, a wireless and low power transceiver 168 and apower supply connector 161.

A low power microprocessor advantageously is a microprocessor thatoperates consuming less than 200 mW, preferably less than 100 mW.

As shown above with reference to tables 6-8, the apparatus 200 of theinvention enables power levels of 100-200 mW to be obtained forreasonably low cable currents (e.g., 100-150 A), that can be ofpractical utility for supplying the monitoring nodes 100.

The low power transceiver 168 comprises an antenna system forreception/transmission of RF signals. Moreover, it is adapted to convertRF signals received by the antenna system into electric signals and toconvert electric signals into RF signals to be transmitted by theantenna system.

For example, the microprocessor and the transceiver can be integrated ina 2.4 GHz XBee module from the company Digi International.

The power supply connector 161 is adapted to be connected to the twoelectric terminations of the winding of one of the apparatuses 200.

The function of the backup battery 164 advantageously is that ofaccumulating the electric energy generated by the apparatus 200, when itexceeds the energy necessary to supply the monitoring node 100, and tosubsequently supply the monitoring node 100 with the accumulated energy,in case of future need (for example when no current or low current isflowing along the core 14). In this embodiment, in case of failure ofthe backup battery 164, the monitoring node can continue being suppliedby the apparatus 200 each time a minimum current is flowing along thecore 14.

The electronic board 160 advantageously further comprises a protectionsystem (not shown) to prevent damage from any high voltages and/or highcurrents that may be induced during short circuits of the power lines. Aprotection system can comprise at least one surge arrester. Moreover, inorder to prevent damage from any high voltages and/or high currents thesupply connector 161 preferably has two pins spaced of at least 5 mm.The electronic board 160 advantageously further comprises a rectifiercircuit (not shown) that converts the alternating current (AC) comingfrom the apparatus 200 to direct current (DC), which is suitable forbeing used by the various components of the electronic board 160.

According to an embodiment (not shown), the apparatus 200 can beconnected to the electronic board through a rectifier circuit and abattery.

In this case, the electronic board is supplied with the intermediationof the battery, the function of the rectifier circuit being that ofconverting the alternating current (AC) coming from the apparatus 200 todirect current (DC), which is supplied to the battery.

However, in this embodiment, in case of failure of the battery, themonitoring node stops to be supplied by the apparatus 200, until thebattery is replaced or repaired.

The microprocessor 162 is adapted to acquire information from thedifferent sensors 169 connected to the connectors 166.

The sensors 169 are adapted to measure at least one parameter of thecable system (e.g., of the core 14).

The sensors may be of the type known to detect, for example, ambienttemperature, ambient humidity, surface cable temperature, water flood,cable current and other parameters of interest, especially forevaluating the overall performance of the cable system.

Advantageously, each monitoring node 100 is adapted to alternativelyoperate according to a sleeping mode and an active mode.

During sleeping mode, the monitoring node is in a idle state wherein noreception, transmission and acquisition operations are performed.

In active mode, the microprocessor 162 of the monitoring node 100 isadapted to acquire the information measured by the various sensors 169connected to the connectors 166 and to convert said information so as togenerate output data adapted to be transmitted by the transceiver 168,according to a determined communication protocol.

In active mode, the microprocessor 162 is also adapted to receive fromthe upward monitoring node, through the transceiver 168, the output datagenerated by the upward monitoring node and by other upward monitoringnodes of the cascade, if any.

Moreover, in active mode, the microprocessor 162 is also adapted to sendto the downward monitoring node, if any, through the transceiver 168,the output data received from the upward monitoring node and the outputdata generated in the monitoring node itself.

For example, in the embodiment shown in FIG. 6, node 0, which is thefirst node of the cascade, is adapted, in active mode, to acquire theinformation measured by its own sensors; to convert said informationinto suitable output data; and to transmit said output data to node 1.

Node 1, when in active mode, is adapted to receive the output data fromnode 0; to acquire the information measured by its own sensors; toconvert said information into suitable output data; and to transmit tonode 2 both the output data received from node 0 and the output datagenerated by node 1 itself.

Node 2, when in active mode, is adapted to receive the output data fromnode 1 (that comprise both the data generated by node 0 and the datagenerated by node 1); to acquire the information measured by its ownsensors; to convert said information into a suitable output data; and totransmit to node 3 both the output data received from node 1 and theoutput data generated by node 2 itself.

Nodes 3 and 4 will act in a way similar to node 2.

Node 5, which is the last node of the cascade, is adapted, when inactive mode, to receive the output data from node 4 (that comprise allthe output data generated by node 0 to node 4); to collect theinformation acquired by its own sensors; to convert said informationinto a suitable output data; and to transmit to the central unit 12 boththe output data received from node 4 and the output data generated bynode 5 itself.

The central unit 12 is adapted to receive from the last node (e.g. fromnode 5) the output data generated by all monitoring nodes 100, and toprocess said output data so as to send them, through a modem or router,to the remote processing station 10, according to a predeterminedcommunication protocol.

In its turn, the remote processing station 10 is adapted to process,according to conventional techniques, the data received from the centralunit 12 and to perform data storing, analysis, visualization (typicallyusing a human readable interface) and alarm generation, when required.Advantageously, the remote processing station 10 is adapted to identifythe data coming from each single sensor of each single node; to fixgiven limits for each sensor; and to automatically generate a specificalarm when a limit of one of the sensors is exceeded. Alarms may betransmitted by e-mail, SMS (Short Message Service) messages, phonecalls, and similar.

Accordingly, the output data generated by the various monitoring nodes100 are collected by the central unit 12 by making the output data passfrom one monitoring node to another, by starting from the firstmonitoring node till the last monitoring node of the cascade. In itsturn, the last monitoring node is connected to the central unit 12 so asto send to it the output data generated by all monitoring nodes 100.

In this way the length of the data links used to transmit the outputdata is reduced with respect to a system wherein each node is directlyconnected to a central controller, which is positioned at the end of thesequence of monitoring nodes 100.

In order to avoid the loss of important information and in order tominimize the waiting time of a monitoring node for receiving the outputdata from an upward monitoring node, the monitoring nodes advantageouslyalternatively operate in a sleeping mode and in an active mode accordingto synchronized time frames.

Advantageously, the monitoring nodes are adapted to carry out a processof auto-synchronization and a process for automatically maintaining thesynchronization.

According to an embodiment of the process of auto-synchronization, whenthe monitoring nodes are not synchronized (for example, when themonitoring system starts working for the first time or when the internalclock of a monitoring node works not properly), the first monitoringnode of the cascade (e.g., node 0) is adapted to alternatively operatein a sleeping mode and in an active mode with a period T (whichindicates the time between the beginning of two consecutive activemodes) while the other monitoring nodes (e.g., node 1, 3, 4 and 5)operate with a period T1. In order to facilitate the synchronizationprocess, T1 is preferably lower than T. For example, T=6 seconds andT1=5 seconds. Moreover, all monitoring nodes initially remain in activemode for a time Ta. Preferably, Ta<<T1 and T. For example, Ta=100 ms.

Then, as shown in FIG. 9, when in active mode, the first monitoring node(e.g., node 0) starts sending the output data generated by itself to thesecond node (e.g., node 1). If the first monitoring node does notreceive an ACK message (indicating data reception) from the second node,the first monitoring node sends the same data to the second node for anumber of times (e.g., 4 times).

When the second node receives the output data from the first monitoringnode, it starts operating with a period T and sending the output datagenerated by itself, together with the output data received by the firstnode, to the third node (e.g., node 2). If the second monitoring nodedoes not receive an ACK message from the third node, the secondmonitoring node sends the same data to the third node for a number oftimes (e.g., 4 times).

When the third node receives the output data from the second monitoringnode, it starts operating with a period T and sending the output datagenerated by itself, together with the output data received by thesecond node, to the fourth node (e.g., node 3).

The above process is continued till also the last monitoring node issynchronized (e.g., node 4 in FIG. 3).

Once synchronized, the monitoring nodes operate in sleeping mode andactive mode with a period T.

During active mode, each monitoring node first waits to receive outputdata from the upward monitoring node. Then—after receipt of the outputdata—it transmits them to the downward monitoring node together with theoutput data generated by itself.

FIG. 10 a shows an embodiment of the process for automaticallymaintaining synchronization between monitoring nodes. At block 400monitoring node N passes from a sleeping mode to an active mode andwaits for reception of output data from upward monitoring node N−1. Atblock 401 the monitoring node N checks output data reception from theupward monitoring node N−1.

If no output data are received within a time Ta, then at block 408 themonitoring node passes in sleeping mode till time T1 lapses, startingfrom the moment the monitoring node has woken up at block 400.

If output data are received within time Ta, then the monitoring node Nsends to the downward monitoring node N+1 the output data received fromthe upward monitoring node N−1, together with the output data generatedby itself (block 402).

Preferably, Ta<<T1, T. For example, Ta=100 ms.

After sending the output data, at block 403 the monitoring node N checksreception of an ACK message from the downward monitoring node N+1.

If the ACK message is received, the procedure passes to block 405.

If no ACK message is received, at block 404 the monitoring node N checksif a maximum number Max (e.g., Max=4) of attempts to send the outputdata to the downward monitoring node N+1 has been exceeded. In thenegative case, the procedure goes back to block 402. In the positivecase, the procedure passes to block 409.

At block 405 the monitoring node N is advantageously adapted to check ifa number R is higher than 1, wherein number R indicates the number ofattempts made by the monitoring node N−1 for sending the output data tomonitoring node N, before monitoring node N receives the output data atblock 401.

If R>1, then at block 406 the monitoring node N passes in sleeping modetill time T−T_(inc) lapses, starting from the moment the monitoring nodehas woken up at block 400.

If R<1 (e.g., if R=0), then at block 407 the monitoring node N isadvantageously adapted to check the time Tw lapsed between the momentthe monitoring node N has woken up at block 400 and the time themonitoring node N has received the output data from the upwardmonitoring node N−1 at block 401.

If waiting time Tw is higher than a predetermined threshold (Th), atblock 410 the monitoring node N passes in sleeping mode till a timeT+T_(inc) lapses, starting from the moment the monitoring node has wokenup at block 400.

If waiting time Tw is not higher than the predetermined threshold (Th),the procedure passes to block 409.

At block 409 the monitoring node N passes in sleeping mode till a time Tlapses, starting from the moment the monitoring node has woken up atblock 400.

For example, Th is equal to 5 ms.

Preferably, Tinc<<Ta. This allows the synchronization maintenanceprocess of the monitoring node N to be performed in little steps that donot compromise the synchronization of the other monitoring nodes. Forexample, Tinc is equal to 1 ms.

The check at block 405 has the purpose of minimizing the number ofattempts made by the monitoring node N−1 for sending output data tomonitoring node N and thus of reducing the power consumption of nodeN−1.

The check at block 407 has the purpose of minimizing the waiting time Twfor receiving the output data from the upward monitoring node N−1. Inthis way the duration of an active mode can be advantageously reducedand the power consumption of the monitoring node further reduced.

According to a preferred embodiment, schematically shown in FIG. 10 b,in case the check at block 401 is negative, monitoring node N is alsoadvantageously adapted to check at block 401′ if the number of attemptsperformed in order to receive output data from the directly upwardmonitoring node N−1 is lower than an upper limit UL. In the positivecase, the procedure passes to block 408. In the negative case, beforepassing to block 408, at block 401″ monitoring node N is configured soas to enable it to receive output data from the upward monitoring nodeN−2.

Even if not shown, the same procedure might be extended to cover alsothe case in which the number of attempts performed in order to receiveoutput data from the upward monitoring node N−2 has reached an upperlimit UL, and so on.

It is noted that, for the sake of expediency, in FIG. 10 b blocks 402 to407 are not shown.

The embodiment of FIG. 10 b advantageously allows automatically copingwith a possible failure of a node of the cascade so that the datacollection process can proceed even in case of a node failure.

In the preferred embodiment of FIG. 10 b, the monitoring nodes and thedata links will be configured so as to enable a monitoring node N toreceive data from at least one monitoring node N−2 preceding thedirectly upward monitoring node N−1.

Advantageously, in case of failure of a monitoring node, the remoteserver—detecting an absence of data coming from said node—can be adaptedto generate a suitable alarm.

In view of the above description, it will be clear that in the presentdescription and claims the expressions “upward monitoring node” and“downward monitoring node” are advantageously used to indicate the firstworking (not failed) upward monitoring node and the first working (notfailed) downward monitoring node, respectively. Similarly, theexpression “last monitoring node” is advantageously used to indicate thelast working (not failed) monitoring node of the cascade.

In the embodiment shown in FIG. 6, the monitoring nodes 100 communicatewith each other through RF wireless data links.

Also the last monitoring node 100 (node 5) and the central unit 12communicate with each other through a RF wireless data link.

RF data links are typically advantageous compared to wired link becausethey reduce installation times and costs.

According to an embodiment, the cable comprising the core 14 is aterrestrial cable. In order to enable the use of RF data links betweenthe monitoring nodes 100, the terrestrial cable is advantageouslypositioned in tunnels.

For example, communications over the RF data links are performedaccording to a standard protocol such as the IEEE 802.15.4 protocol,operating at 2.4 GHz.

According to this protocol, data are sent through 123 bytes data framesand time multiplexing is used to put the data of each monitoring node inthis data frame, according to techniques well known in the art.

In particular, each data frame will be generated by the first node andeach monitoring node will be adapted to put its own output data into thedata frame received from the upward monitoring node and to transmit thedata frame, containing its own output data and the output data of theupward monitoring nodes, to the next node until the last node isreached. Moreover, each monitoring node, before transmitting the dataframe, will be adapted to update a “sender address” field of the dataframe in order to identify itself (e.g., by using a suitable identifier)as sender of the data frame in place of the upward monitoring node fromwhich it has received the packet.

FIG. 11 shows an example of a 123 bytes data frame containing 10 packets(from 0 to 9), each of 12 bytes in length; a frame terminator of 2 bytesin length; and a sender address of 1 bytes in length. The frameterminator indicates the end of a data frame, while the sender addressis adapted to contain the address of the current node sending the dataframe to a downward node.

Of course, data frames of more or less than 123 bytes may be used.

Each packet can comprise, for example, actual values of the parameterssensed by the sensors of the monitoring node; service information (asinformation indicating what subset of nodes can insert data into thecurrent data frame; the above mentioned R number, indicating the numberof attempts made by the monitoring node for sending the output data tothe downward monitoring node; and similar); and data indicative ofquality of data/ACK transmissions between nodes.

In a system with more than 10 monitoring nodes, it can be provided thatonly a subset (comprising at most 10 monitoring nodes) at a time isadapted to put its own output data into a data frame. For example, with20 monitoring nodes, it can be provided that at a first time, onlymonitoring nodes 0 to 9 put their own output data into a data frame,nodes 10 to 19 only propagating one another the data frame till the lastmonitoring node. At a second time, nodes 0 to 9 will only propagate oneanother the data frame, while node 10 to 19—besides propagating oneanother the data frame till the last monitoring node—also put their ownoutput data into the data frame. As disclosed above, the informationabout what subset of nodes can insert data into the current data framewill be contained in the data frame, as service information. Moreover,each node, at its turn, put its own output data into a correspondingpacket of the data frame (e.g., node 0 into packet 0, node 1 into packet1 and so on).

The central unit 12 and the remote processing station 10 can communicatewith each other through a data link at least in part wireless.

For example, communications between the central unit 12 and the remoteprocessing station 10 are in part performed through a GSM/GPRS network1.

Even if not shown, the apparatuses of the invention for generatingelectric energy can also be used in a substation (e.g., an urbansubstation) comprising terminal parts of a plurality of cable systemsbelonging to different electric power transmission systems (wherein, forexample, each cable system is a three-phase system comprising at leastthree insulated, individually sheathed, electric conductors). In thiscase, the cascade of monitoring nodes can be mounted in the substationso as to monitor the terminal parts of the plurality of cable systemsand the apparatuses of the invention can be used to electrically supplyat least part of said monitoring nodes. The monitoring nodes can, forexample, be mounted so that each terminal part to be monitored iscoupled to at least one of the monitoring nodes of the cascade.

In an embodiment (not shown), the monitoring nodes 100 can communicatedata with each other according to a PLC (Power Line Communication)technology, by exploiting the screening layer of the core 14 (forexample the metal screen 145 of FIG. 5). In particular, each monitoringnode 100 can be provided with an electro-magnetic transceiver comprisinga coil. In this way, an alternating current flowing along the coil willproduce a magnetic field that induces a varying voltage in the screeninglayer of the core 14. In its turn, an alternating current flowing alongthe screening layer of the core 14 will produce a magnetic field thatinduces a varying voltage in the coil of the electro-magnetictransceiver of the monitoring node.

This embodiment can be particularly useful when RF communications cannotbe used as, for example, in case of buried terrestrial cables.

1-17. (canceled)
 18. An apparatus for generating electric energy in anelectric power transmission system, comprising: an arc shapedferromagnetic body extending along a longitudinal axis; and at least oneelectrically conducting winding wound around the ferromagnetic body toform turns in planes substantially perpendicular to the arc.
 19. Theapparatus according to claim 18, wherein said arc extends at an angleless than 300°.
 20. The apparatus according to claim 18, wherein saidarc extends at an angle at least equal to 45°.
 21. The apparatusaccording to claim 18, wherein said arc extends at an angle at leastequal to 180°.
 22. The apparatus according to claim 18, wherein theferromagnetic body has a length from 6 cm to 40 cm.